18 
CRYSTALLIZATION IN ROCKS. 
Jan., 1890 . 
The formation of globulites was studied by Vogelsang in 
mixtures of sulphur and Canada balsam dissolved in bisulphide 
of carbon. As the solvent evaporates the sulphur tends to 
crystallize, but the viscidity of the Canada balsam prevents 
this taking place at once, and globulites are formed which 
apparently remain fluid for some time and can be observed to 
take on crystalline form at the instant of combining with a 
crystal in process of formation.* 
These elementary forms when further grouped together 
form minute bodies which are now in many cases recognizable 
as crystals, though it may be impossible to determine 
their species with certainty. These are grouped under 
the general name of microlites, and they can frequently be 
detected as it were in the very act of building up a recogniz¬ 
able crystal. This larger crystal may under such circumstances 
consist of a mere skeleton of the actual mineral, while a 
great part of the enclosed space is occupied by the glassy 
ground mass of the rock. 
It is the microlites, with their wonderful grouping and 
suggestive relationships, which give so much charm to the 
glassy rocks. The grouping in many cases is very beautiful, 
and perhaps of all the beautiful examples the most lovely are 
to be found in the fernlike groups of hornblende microlites 
which occur in some of the pitchstones of Arran. A very 
noticeable feature in this rock is the clear colourless space 
round each group. The fine dusty material which fills the 
main mass of the glass, and which is seen under high powers 
to be composed of extremely small needles, felted together in 
such quantity as to make parts quite opaque, has evidently 
been used up in the formation of the aggregates of microlites. 
A somewhat similar phenomenon occurs round the splierulites 
in the artificial glassy basalt produced by Messrs. Chance, of 
Oldbury, some years ago by melting Rowley rag. Where the 
splierulites have been formed in the process of annealing, 
they are surrounded by a space of which the colour is very 
much lighter than that of the very dark brown glass, which 
is the general result of the experiment. 
Such specimens as the two last named show us very 
plainly what an amount of molecular mobility there must be 
in a mass which is already cool enough to be practically 
solid. We may group such observations with others on the 
annealing and devitrification of glass, where, without the 
shape of the mass being at all altered, there has been 
sufficient molecular freedom produced to allow of considerable 
* See Vogelsang’s “ Die Krystalliten.” 
